The actual Northrop Grumman facility is in St. Johns County proper, outside of the St. Augustine city limits. SJC is the one that is granting the incentives and is the real player financially down there. SJC also recently gave Advanced Disposal a nice incentive package to relocate.
SJC now has quite a bit of the metro's middle class, upper middle class, and wealthy residents, and therefore can not only afford to have a great school system, but also make economic development deals like this (doesn't hurt that they also get impact fees for new development). In fact, SJC is #2 in the state for per capita income, #1 in median household income, and #1 in median family income. A big focus of economic development efforts there is attracting employers and retail to match residential. I suspect we'll be seeing a lot more deals like this and Bass Pro Shops in the future.